1. Field of the Invention
The present invention relates to an organic light emitting device (OLED) in which the anode includes a layer of gold or gold composition under a thin layer of a functional charge transporting thiol or thiol derivative.
2. Description of the Related Art
Significant efforts have been expended in developing suitable materials for use in organic light emitting devices (OLEDs). Such devices are commercially attractive because they offer the promise of low-cost fabrication of high-density pixeled displays exhibiting bright electroluminescence with long life times and wide color range.
A typical OLED is fabricated by sandwiching an emissive layer between an anode and a cathode. When a bias is applied across the electrodes, holes and electrons are respectively injected from the anode and cathode into the emissive layer, typically facilitated by hole transport and electron transport layers adjacent to the respective electrodes. The holes and electrons radiatively combine in the emissive layer and emit light. Improved performance can be obtained if blocking layers are provided to block against the injection of either holes or electrons from the adjoining layer and their subsequent escape from the device. Some of these layers can be combined. For example, a double-layered structure is fabricated from a combined hole-injecting and transporting layer together with a combined electron-transporting and light-emitting layer. Likewise, a triple-layered structure is composed of a hole-injecting and transporting layer, a light-emitting layer, and an electron-injecting and transporting, layer.
Light emission from the emissive layer must be emitted from the OLED to be visible, and at least one of the anode and cathode must therefore be sufficiently transparent in the visible light range (a visible light emission transparency greater than 60%). Indium tin oxide-coated glass (herein after “ITO-coated glass”) is usually used as the anode, based on its good conductivity, high transparency (larger than 90% at 550 nm), and high work function (around 4.8 eV) which allows good hole injection. Its electrical properties have been described in many references (such as I. Hamberg and C. G. Granqvist, J. AppI. Phys., 60, R123 (1986)).
Despite its widespread use, ITO-coated glass has its drawbacks. For example, there is often a large variation in composition (that is, the ratio of tin oxide to indium oxide). This variation in composition may introduce variations in work functions and actually form barriers to hole injection rather than facilitating it. ITO-coated glass also is sensitive to electrochemical changes under high electrical fields which are ordinarily present during use of the OLED, and sometimes present during its fabrication. The ITO surface is often rough, since it is formed by sputtering and is subject to various surface physical changes by processing and fabrication steps such as cleaning, drying, and plasma/oxygen cleaning.
Because of these and other drawbacks, the prior art has investigated various modifications to ITO-coated glass. One group has investigated a modification through the incorporation of a thin layer of phthalocyanines (around 15 nm thick) in contact with the ITO (S. A. Van Slyke, C. H. Chen and C. W. Tang, Appl. Phys. Lett., 69, 2160 (1996)). Another group has investigated the treatment of ITO with a conducting polymer such as poly(3,4-ethylene dioxythiophene) (PEDOT) (see J. Appl. Phys., 84 (12), 6859 (1998)).
Still other approaches have focused on the hole transporting layer that often accompanies OLEDs, and various organic hole transporting compounds have been used to enhance the effect of the ITO-coated glass anode. However, because an interface is necessarily formed between the ITO and the organic hole transporting layer, other disadvantages might result, such as the formation of voids and a breakdown in the interface. In addition, for top-emission OLEDs (that is, OLEDs in which light is emitted through the cathode rather than the anode), the hole transporting layer is unable to assist in reflection of light to the anode.